The possibility of terrorists using drones to drop explosives or even unleash biological attacks on U.S. soil is a growing threat, Nicholas Rasmussen, the director of the National Counterterrorism Center, told a Senate panel Wednesday.

“Two years ago this was not a problem,” he told the Senate Committee on Homeland Security and Governmental Affairs. “A year ago this was an emerging problem. Now it’s a real problem.”

The counterterrorism center is coordinating with federal and local law enforcement and aviation regulators to study the issue and potential countermeasures, he said. (The video below shows security officials briefing the committee on the risk.)

“There is a community of experts who is focused on this in the federal government, who is focused on this pretty full time,” he said.

The federal government has already invested in technology to interfere with radio transmissions to control drones, and the Department of Defense issued policies this year allowing military sites to shoot down or disable drones coming too close to its facilities. The Army has also banned the use of drones from Chinese manufacturer DJI amid security concerns.

Quadcopters by DJI, the world’s largest consumer drone maker, have also been implicated in a surge of attacks in Iraq and Syria in recent years (see the New York Timesvideo below). The threat overseas has led the Pentagon to recently spend $700 million on an anti-drone effort drawing on the resources of the military, Silicon Valley, and defense giants like Boeing and Raytheon. We’ve written about some efforts at anti-drone defenses, and the technology and politics involved; you can read more here and here.